1. Field of the Invention
The present invention relates to a DC offset cancellation circuit that cancels a DC offset voltage occurring between a pair of complementary differential output signals outputted from a differential amplification circuit, a differential amplification circuit with a DC offset cancellation circuit, and a photo-electric pulse conversion circuit that uses the differential amplification circuit capable of DC offset cancellation to convert an optical pulse signal to a corresponding electrical pulse.
Alternatively, it relates to a pulse shaping circuit that generates a shaped pulse signal whose logic changes in a manner similar to a rise and a fall of a base square-wave pulse signal, a pulse generation circuit that uses this pulse shaping circuit, and a photo-electric pulse conversion circuit that uses the pulse shaping circuit to convert an optical pulse signal to a corresponding electrical pulse.
2. Description of Related Art
(Related Art 1)
In a differential amplification circuit that amplifies an input signal and outputs a pair of differential output signals, a difference in reference voltages (hereinafter referred to also as a DC offset voltage) occurring between a pair of complementary differential output signals outputted from the differential amplification circuit, namely, between a non-inversion output signal and an inversion output signal presents at times a problem. Therefore, a differential amplification circuit provided with a DC offset cancellation circuit that cancels the DC offset voltage is proposed.
A photo-electric pulse conversion circuit 10 shown in FIG. 35 will be explained as an example. The photo-electric pulse conversion circuit 10 converts an optical pulse signal LT to an electrical pulse signal xRX. For example, it is used as a receiving circuit in IrDA communications and transmits an inversion electrical pulse signal xRX to a demodulator circuit at a later stage.
When it is used in such optical communications, the distance from a transmitter circuit (a light source) to a receiving circuit (photodiode PD) is not constant, and therefore there are various conditions. In some cases, the received optical pulse signal LT is very feeble due to a long distance, and in other cases, the received signal LT is extremely strong due to a short distance. As a result, a current input signal fluctuates from scores of nA to several mA which is several hundred times as large as the scores of nA. Even in such cases, it is necessary to reliably receive the optical pulse signal, to shape the waveform while accurately maintaining a pulse width thereof, and to send the resultant signal to the demodulator circuit at a later stage.
In the photo-electric pulse conversion circuit 10, when the photodiode PD receives the optical pulse signal LT that rises at a second timing t2 and falls at a first timing t1, a pulsating current signal Iin flows according to the intensity of the light. An I-V conversion circuit IV converts this current signal Iin to a pair of complementary differential voltage signals, namely, a non-inversion voltage signal V1P that is in the same phase as the optical pulse signal LT and the current signal Iin and an inversion voltage signal V1M that is complementary thereto and outputs these signals. The waveforms of the differential voltage signals V1P, V1M when a large signal is inputted are slightly different from those when a small signal is inputted as shown in FIG. 36. When a small signal is inputted, the current signal Iin of the photodiode PD having a pulse width tpw that nearly corresponds to the optical pulse signal LT is obtained. When a large signal is inputted, however, the waveform has a dull rising edge and a dull falling edge though it has a generally square shape. This is because the electrical signal fails to accurately follow changes in the optical input. Particularly, since the falling edge after the first timing t1 falls slowly, the non-inversion voltage signal V1P also falls slowly as shown in FIG. 36.
The differential voltage signals V1P, V1M are then amplified by a first differential amplification circuit AMP1 provided with a DC offset cancellation circuit OFC indicated by dashed lines in FIG. 35 and a second differential amplification circuit AMP2. Then, as shown in FIG. 37, a reference voltage VREF according to an output VO of the amplifier is generated by a reference voltage generation circuit REFG and both signals are compared with each other by a comparison circuit CMP to obtain an inversion electrical pulse signal xRX which has the pulse width tpw corresponding to the optical pulse signal LT and which falls at the second timing t2 and rises at the first timing t1.
More specifically, an offset adding circuit (mixing circuit) OFP is used to mix the offset cancellation voltage VOC into the differential voltage signals V1P, V1M such that a negative feedback is performed, thereby generating second differential signals V2P, V2M which are amplified by the first differential amplification circuit AMP1 to output third differential signals V3P, V3M. In the DC offset cancellation circuit OFC, the third differential signals V3P, V3M are filtered by a low-pass filter LPF having characteristics of a cutoff frequency fc1 and a slew rate SR1 to obtain the offset cancellation voltage VOC. Since the DC offset voltage occurring between the third differential signals V3P and V3M is negatively fed back in this manner, the DC offset voltage between the differential output terminals of the differential amplification circuit AMP1 can be canceled. If a DC offset voltage exists, an output Va from the second differential amplification circuit AMP2 fluctuates causing the pulse width obtained in the comparison circuit CMP to fluctuate. Thus, the pulse width of the inversion electrical pulse signal xRX obtained may become different from the optical pulse signal. By canceling the DC offset voltage, however, the inversion electrical pulse signal xRX having the pulse width tpw which accurately corresponds to the optical pulse signal can be obtained.
To obtain the inversion electrical pulse signal xRX having the accurate pulse width tpw, it is necessary to give the reference voltage VREF an appropriate time constant according to the magnitude of the output VO.
(Related Art 2)
It is possible to employ a differential amplification circuit provided with a DC offset cancellation circuit in the same manner also in a photo-electric pulse conversion circuit 20 with another configuration (see FIG. 39).
The photodiode PD receives the optical pulse signal LT that rises at the second timing t2 and falls at the first timing t1 to provide the current signal Iin also in this photo-electric pulse conversion circuit 20. However, the photo-electric pulse conversion circuit 20 uses, instead of the I-V conversion circuit IV, a differentiating I-V conversion circuit DIV to convert a waveform of the current signal Iin to a pair of complementary differential voltage signals VD1P, VD1M whose waveforms are similar to a differentiated waveform of the current signal Iin. The differential voltage signals VD1P, VD1M are then amplified by the differential amplification circuit AMP provided with the offset cancellation circuit OFC to output third differential signals VD3P, VD3M. The third differential signals VD3P and VD3M are compared with each other by the comparison circuit CMP and the inversion electrical pulse signal xRX is obtained.
In the photo-electric pulse conversion circuit 20, the differential voltage signals VD1P, VD1M whose waveforms are similar to a differentiated waveform of the current signal Iin are obtained, and are then amplified. A third differential signal VD3P and a third differential signal VD3M that sharply fall or rise at the first or the second timing t1, t2 are compared. It is therefore possible to accurately reproduce the pulse width tpw of the optical pulse signal LT in the obtained inversion electrical pulse signal xRX. In addition, the circuit has the advantage that there is no need of separately using the reference voltage generation circuit REFG to generate the reference voltage VREF according to the output VO as in related art 1 (see FIG. 35).
A DC offset voltage VOS of a small value may be added in the comparison circuit CMP to prevent a malfunction caused by noise.
(Related Art 3)
In the circuit shown in related art 2 (see FIG. 39), the pulse signal is once differentiated to obtain the differential signals, and using these differential signals, a pulse signal having the same pulse width as the original pulse signal is obtained. As a circuit of the same type, a photo-electric pulse conversion circuit 30 shown in FIG. 41 may be configured.
Namely, in the photo-electric pulse conversion circuit 30, the optical pulse signal LT that rises at the second timing t2 and falls at the first timing t1 is received by the photodiode PD and the current signal Iin is obtained. Then, the current signal Iin is converted to the corresponding voltage signal V1 and the resultant voltage signal is outputted by the I-V conversion circuit IV. The voltage signal V1 is then amplified by the differential amplification circuit AMP. Thereafter, a differentiating differential amplification circuit DAMP is used to differentiate and amplify the second differential signals V2P, V2M to output third differential signals VD3P, VD3M. In addition, an offset voltage VOS of a small value is added so that the reference voltage of the third non-inversion signal VD3P is relatively lower than the reference voltage of the third inversion signal VD3M. These third differential signals VD3P, VD3M are then compared with each other by the comparison circuit CMP to obtain the inversion electrical pulse signal xRX that falls at the second timing t2 and rises at the first timing t1. As mentioned above, the purpose of adding the offset voltage VOS is to prevent a malfunction caused by noise.
The current signal Iin and the voltage signal V1 have slightly dull waveforms that gradually fall after the first timing t1 (see FIG. 36) also in the photo-electric pulse conversion circuit 30. However, since the third differential signals VD3P, VD3M that sharply rise or fall at the first or the second timing t1, t2 are compared to obtain the inversion electrical pulse signal xRX, it is possible to accurately reproduce the pulse width tpw of the optical pulse signal LT in the inversion electrical pulse signal xRX. Furthermore, the circuit has the advantage that there is no need of separately using the reference voltage generation circuit REFG to generate the reference voltage VREF according to the output VO as in the circuit 10 shown in related art 1 (FIG. 35).
In the photo-electric pulse conversion circuit 10 of related art 1, however, the low-pass filter LPF with the cutoff frequency fc1 is used to provide a negative feedback control of the DC offset voltage. Therefore, not only DC components, but also low-frequency components of AC components contained in the pulse signal waveform are fed back. Namely, as shown in FIG. 38(a), if the DC offset voltage DCO exists between the third differential signals V3P and V3M of the first differential amplification circuit AMP1 and is negatively fed back, DC cancellation components are contained in the offset cancellation voltage VOC outputted from the low-pass filter LPF as shown in FIG. 38(b) and they function to cancel the DC offset voltage DCO. However, since the low-frequency AC components also pass through the low-pass filter LPF as mentioned above, the low-frequency AC components are also superposed on the offset cancellation voltage VOC as shown in FIG. 38(b).
The magnitude of these AC components contained in the offset cancellation voltage VOC gradually increases during a second period d2 from the second timing t2 to the first timing t1. During a first period d1 from the first timing ti to the second timing t2, it gradually decreases to return to an original zero level which is maintained. This is because the AC components are contained in the second period d2 as can be easily understood from FIG. 38(a). The gradient of the graph showing the offset cancellation voltage VOC corresponds to the characteristics of the low-pass filter LPF (the cutoff frequency and the slew rate), and the increasing gradient and the decreasing gradient become almost the same.
If the second period d2 is longer than the first period d1 as shown in FIG. 38(c), the AC components contained in the offset cancellation voltage VOC cannot decrease in the first period by the amount increased in the second period. As a result, the AC components contained in the offset cancellation voltage VOC gradually accumulate as shown in FIG. 38(d) (in this example, they gradually increase). Therefore, as a result of accumulated AC components being negatively fed back, the waveforms of the third differential signals V3P and V3M as the output from the first differential amplification circuit AMP1 are distorted. This could result in a malfunction or other problem when the inversion electrical pulse signal xRX is obtained in the comparison circuit CMP. Moreover, as AC components accumulate, the waveforms are distorted so as to gradually shift downward, and approach an upper limit value or a lower limit value of the third differential signals V3P, V3M. As a result, the dynamic range may become small and the signal amplitude may become small, and in extreme cases, the third differential signals V3P, V3M may disappear.
On the other hand, in the photo-electric pulse conversion circuit 20 according to related art 2, the current signal Iin as shown in FIG. 40(b) flows through the photodiode PD when receiving the optical pulse signal LT with the pulse width tpw as shown in FIG. 40(a). FIG. 40(b) shows a case in which a large optical pulse signal LT with a high intensity is inputted. In the first period d1, the current signal Iin forms a gradually decreasing long tail. The signal is then subjected to differentiation and I-V conversion performed by the differentiating I-V conversion circuit DIV to obtain the non-inversion voltage signal VD1P shown in FIG. 40(c) and the inversion voltage signal VD1M. These signals are then amplified by the differential amplification circuit AMP to obtain the third differential signals VD3P, VD3M [see FIG. 40(d)]. In this example, the amplitudes of the amplified third differential signals VD3P, VD3M are limited by the upper limit value or the lower limit value in the second period d2 and the first half of the first period d1. As a result, their waveforms are not similar to the waveform of the non-inversion voltage signal VD1P shown in FIG. 40(c). Unlike the photo-electric pulse conversion circuit 10 [see FIGS. 37(a) and (c)] this circuit employs a differentiated waveform and therefore the non-inversion voltage signal VD1P swings to the positive and negative directions (upward and downward) with respect to the reference voltage.
The low-pass filter LPF with the cutoff frequency fc1 (slew rate SR1) is used also in the photo-electric pulse conversion circuit 20 according to related art 2. Therefore, not only the DC components but also the low-frequency components of the AC components are fed back. That is, the AC components are superposed on the offset cancellation voltage VOC.
The magnitude of the AC components contained in the offset cancellation voltage VOC gradually increases in the second period d2 from the second timing t2 to the first timing t1 as shown in FIG. 40(e) and gradually decreases in the first period d1 from the first timing t1 to the second timing t2. Unlike the photo-electric pulse conversion circuit 10 [see FIG. 38(b)], however, it continues decreasing in the first period d1. The gradient of the graph showing the offset cancellation voltage VOC corresponds to the characteristics of the low-pass filter LPF (the cutoff frequency and the slew rate), and the increasing gradient and decreasing gradients become almost the same.
If the first period d1 is not equal to the second period d2 (if the duty ratio of the pulse is not 50%), the offset cancellation voltage VOC gradually fluctuates. If d1>d2 as shown in FIG. 40, for example, the AC components contained in the offset cancellation voltage VOC gradually accumulate, causing the offset cancellation voltage VOC to gradually diminish as shown in FIG. 40(e).
As a result, the third differential signals VD3P, VD3M of the differential amplification circuit AMP are distorted and the third non-inversion signal VD3P approaches the upper limit value as shown in FIG. 40(f), for example. Therefore, a malfunction may occur when obtaining the inversion electrical pulse signal xRX in the comparison circuit CMP. Moreover, the waveforms approach the upper limit value or the lower limit value of the third differential signals VD3P, VD3M. As a result, the dynamic range may become small and the signal amplitude may become small and, in extreme cases, the third differential signals VD3P, VD3M may become disappear.
In the photo-electric pulse conversion circuit 30 according to related art 3, when the pulse width tpw of the base pulse signal, that is, the optical pulse signal LT becomes long, the gradually downward-going third non-inversion signal VD3P and the gradually upward-going third inversion signal VD3M can cross at a time tx after the second timing t2 as shown in FIG. 42. Then, the inversion electrical pulse signal xRX which is the output of the comparison circuit CMP is inverted. Therefore, as shown in the lower part of FIG. 42, there arises a problem that the pulse width of the inversion electrical pulse signal xRX, which should rise at the first timing t1, becomes shorter. Particularly, the pulse width tends to become shorter when the offset voltage VOS is made greater in an attempt to prevent a malfunction caused by noise.
If the comparison circuit CMP is given hysteresis characteristics (hysteresis voltage Vh) so as to satisfy Vh> VOS as shown in FIG. 43, the third non-inversion signal VD3P and the third inversion signal VD3M do not cross between the second timing t2 and the first timing t1, and thus the correct pulse width tpw is obtained in the inversion electrical pulse signal xRX.
In the case where an arrangement is made to satisfy Vh>VOS as described above and once the third inversion signal VD3M becomes smaller than the third non-inversion signal VD3P when the circuit is started or noise intrudes, the inversion electrical pulse xRX which is the output of the comparison circuit CMP is inverted, that is, the level of the inversion electrical pulse xRX becomes LOW level as shown in FIG. 44. This also causes the same effect as relatively decreasing the third non-inversion signal VD3P by the amount equal to hysteresis voltage Vh. Since the third inversion signal VD3M becomes smaller than the third non-inversion signal VD3P, the inversion electrical pulse signal xRX is fixed to LOW level even after the noise has disappeared and the inversion electrical pulse signal xRX does not fall at the second timing t2. It thereafter returns to HIGH level at the first timing t1. In this case, therefore, the optical pulse signal has not been properly received.
Thus, in the photo-electric pulse conversion circuits 20, 30 according to related arts 2, 3, it is difficult to prevent a malfunction due to noise or the like by setting the offset voltage VOS and the hysteresis voltage Vh to adequate values simultaneously.